Interference colored titanium with protective oxide film

ABSTRACT

Coatings for titanium and titanium alloy substrates are described. The coatings can include an aluminum oxide layer and, in some cases, a thin titanium oxide layer. If the coating includes a titanium oxide layer, the aluminum oxide layer can cover and protect the titanium oxide layer from abrasion and scratching. In some examples, the titanium oxide layer has a thickness sufficient to provide a color by thin-film interference, which can be visible through the overlying aluminum oxide layer. In some embodiments, the aluminum oxide layer is colorized using an anodic dye, pigment or metal colorant, which can combine with interference colors of the titanium oxide layer.

FIELD

The described embodiments relate to oxide coatings for titanium and titanium alloys. The coatings can include a titanium oxide film having a color produced by thin-film interference, and a protective aluminum oxide film.

BACKGROUND

Titanium and its alloys are known for their high strength, low density and corrosion resistance. For these reasons, titanium and titanium alloys are utilized in a number of applications that require a strong, lightweight and corrosion resistant metal. Although titanium surfaces are regarded as inherently corrosion resistant, they can readily oxidize, which makes them susceptible to staining when exposed to certain environments. Furthermore, when titanium is anodized, a thin barrier layer type film is formed, which offers little abrasion protection. In particular, the titanium oxide film it very thin and therefore susceptible to uneven wearing, which can result in a cosmetically unappealing titanium part. What are needed, therefore, are coatings for titanium and titanium alloys that offer improved abrasion resistance and that are cosmetically appealing.

SUMMARY

This paper describes various embodiments that relate to titanium and titanium alloys. In particular embodiments, methods of providing abrasion resistant and cosmetically appealing oxide coatings on titanium substrates are described. In some cases, the coatings are characterized as having a color produced by thin-film interference effects.

According to one embodiment, a part is described. The part includes a substrate composed of titanium or a titanium alloy. The part also includes a coating disposed on the substrate. The coating includes an aluminum oxide layer. The coating can also include a titanium oxide layer.

According to another embodiment, a method of forming a coating on a substrate is described. The substrate is composed of titanium or a titanium alloy. The method includes depositing a layer of aluminum or an aluminum alloy on the substrate. The method also includes converting at least a portion of the layer of aluminum or the aluminum alloy to an aluminum oxide layer. In some cases, the method further includes converting a portion of the substrate to a titanium oxide layer.

According to a further embodiment, an enclosure for an electronic device is described. The enclosure includes a substrate composed of titanium or a titanium alloy. The substrate has a coating corresponding to an outer surface of the enclosure. The coating includes an aluminum oxide layer. In some cases, the coating also includes a titanium oxide layer.

These and other embodiments will be described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements.

FIG. 1 shows perspective views of devices having metal surfaces that can be treated with the coatings described herein.

FIGS. 2A-2D show cross-section views of a part undergoing a surface coating process, in accordance with some embodiments.

FIGS. 3A-3C show cross-section views of an interface portion of the part of FIGS. 2A-2D, in accordance with some embodiments.

FIG. 4 shows a chart showing a relationship between anodizing voltage potential and interference color of a titanium oxide layer, in accordance with some embodiments.

FIGS. 5A and 5B show a plan view and a partial cross-section view of a part having a coating that includes a portion that includes interference coloring by a titanium oxide layer.

FIG. 6 shows a flowchart indicating a process for forming a protective oxide coating on a titanium substrate, in accordance with some embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims.

Titanium and its alloys can form strong, lightweight structures, but offer limited options for surface finishing. Described herein are coatings for titanium and titanium alloy substrates that are durable, stain resistant and cosmetically appealing. The coatings can include an aluminum oxide layer, and in some embodiments, include a titanium oxide layer. The thicker and more scratch resistant aluminum oxide layer is disposed over the thinner titanium oxide layer so as to protect the titanium oxide layer from abrasion and scratching. In some cases, the thickness of the titanium oxide layer is tuned to cause thin-film interference coloring effects, thereby imparting a color to the titanium part. The overlying aluminum oxide layer can protect the integrity of the titanium oxide layer, thereby preserving the interference coloring. Compared to colors provided by some conventional oxide coloring techniques, interference colors of the titanium oxide layer are stable when exposed to ultraviolet (UV) light.

The coating can be formed using a number of techniques. In some embodiments, this involves applying an aluminum layer on the titanium substrate, then anodizing the aluminum layer. In some cases, the entire aluminum layer is converted to a corresponding aluminum oxide layer, and a portion of the titanium substrate is also converted to a titanium oxide layer. In some applications, the anodizing conditions are chosen so as to yield an optically clear, porous oxide film (for example, a “Type II” anodic oxide as defined by Mil-A-8625 specifications). This can permit optimal viewing of the underlying interference colored titanium oxide layer. The anodizing process can also be chosen so as to provide a hard aluminum oxide layer that protects the underlying titanium oxide layer from scratching and abrasion.

The substrates and coatings described herein are well suited for providing cosmetically appealing consumer products. For example, the methods described herein can be used to form durable and cosmetically appealing finishes for housing of computers, portable electronic devices, wearable electronic devices, and electronic device accessories, such as those manufactured by Apple Inc., based in Cupertino, Calif. As used herein, the terms oxide, oxide coating, oxide film, oxide layer, etc. can be used interchangeably and can refer to suitable oxide material, unless otherwise specified.

These and other embodiments are discussed below with reference to FIGS. 1-6. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting.

FIG. 1 shows consumer products than can include titanium or titanium alloy substrates described herein. FIG. 1 includes portable phone 102, tablet computer 104, smart watch 106 and portable computer 108, each of which can include enclosures that are made of metal or have metal sections. These metal or metal sections can be composed of titanium or titanium alloys in order to provide high strength and stiffness. Titanium has a high specific strength, thus the enclosures made of titanium and its alloys can be more lightweight with the same strength compared to enclosures made of other metals. Titanium surfaces also present good hardness, which generally translates to good scratch and abrasion resistance in everyday use. For example, Ti-6Al-4V titanium alloy has a hardness of about 350 HV (per Vickers hardness test), which is in itself comparable to the surface hardness of a typical anodized aluminum surface (about 300-400 HV as measured at 50 gram load).

In spite of these advantages, surfaces of titanium can be less abrasion resistant and scratch resistant compared to, for example, anodized aluminum surfaces. Conventional anodizing of titanium does not offer the thick, clear and hard oxides, which are useful in the protection of cosmetic aluminum surfaces—but instead forms thin, interference colored barrier layers, which offer little abrasion protection and can remain vulnerable to chemical staining. Even slight changes in the thickness of these films (e.g., about 10 nm) can result in significant changes in the interference coloring, and hence obvious changes in surface appearance after even light abrasion.

Another problem associated with titanium surfaces relates to corrosion. Whilst titanium and its alloys are noted for excellent corrosion resistance, their surfaces readily oxidize. Thus, titanium surfaces can be prone to staining and tarnishing under exposure to certain environments. Although generally limited to surface of the titanium or titanium alloy, and often only to a limited area of the surface, these stains can significantly and permanently change the local visual appearance of the surface. Again, the extremely thin oxide films formed by conventional anodizing of titanium do not provide significant protection against this effect, and can even exacerbate it due to the greater optical contrast that varying oxide film thicknesses and chemistries of distinct colors can give.

The coatings described herein can be applied to surfaces of titanium or titanium alloys in order to improve their scratch resistance and retention of good cosmetic quality. Thus, the coatings are well suited for coating titanium or titanium alloy portions of consumer products such as enclosures of devices 102, 104, 106 and 108, which are subject to abrasion and scratching during normal use. In some cases, the coatings are applied to exterior portions of the enclosures such that the coatings are visible and tactilely accessible to users of devices 102, 104, 106 and 108.

FIGS. 2A-2D show cross-section views of part 200 undergoing a surface coating process in accordance with some embodiments. Part 200 can correspond to a portion of an enclosure, such as enclosures for devices 102, 104, 106 and 108. FIG. 2A shows substrate 202, with surface 204 corresponding to a surface undergoing treatment. In some embodiments, substrate 202 is composed of titanium or a titanium alloy. In some cases, substrate 202 is composed of a Ti-6Al-4V titanium alloy. In some cases, substrate 202 is composed of a custom titanium alloy having alloying elements in predetermined amounts for providing certain benefits, such as high strength, corrosion resistance or cosmetic quality. In other embodiments, substrate 202 is composed of zirconium, zirconium alloys, tantalum, tantalum alloys, hafnium, hafnium alloys, niobium, or niobium alloys. In some cases, substrate 202 includes titanium and one or more of zirconium, tantalum, hafnium and niobium.

At FIG. 2B, substrate 202 has optionally undergone one or more surface treatment processes. For example, surface 204 can be planarized to be substantially flat, or can be machined to have curved surface profile. In some embodiments, surface 204 is polished. Additionally or alternatively, surface 204 can undergo a texturing process, such as one or more of a blasting and chemical etching process. In some embodiments, a very thin metal intermediate layer (e.g., about 2-6 nanometers of silver) is deposited on surface 204 prior to a subsequent step of depositing an aluminum layer in order to enhance the adhesion of the aluminum layer to substrate 202. Since metals such as silver are inert with respect to an anodizing process and the intermediate layer would nominally be very thin, such an intermediate metal layer may be beneficial in some cases.

At FIG. 2C, aluminum layer 206 is deposited on surface 204 of substrate 202. In some embodiments aluminum layer 206 is composed of an aluminum alloy, such as a 6000 or 7000 series alloy. In some embodiments, aluminum layer 206 is composed of substantially pure aluminum, which generally results in a more transparent and colorless aluminum oxide layer compared to aluminum alloys. Aluminum layer 206 can be deposited using any suitable process For example, aluminum layer 206 can be deposited using a physical vapor deposition (PVD), hot dip aluminizing, cladding and/or electroplating processes. If a hot dip aluminizing process is used, it may be necessary to remove some of the aluminum material using, for example, a chemical etching process, in order to achieve an appropriate thickness 214. If a conformal deposition technique is used, the geometry of surface 208 of aluminum layer 206 can correspond to the geometry of surface 204 of substrate 202. For example, a conformal deposition process can result in flat, curved, polished and/or textured surface 208 of aluminum layer 206 to match corresponding flat, curved, polished and/or textured surface 204 of substrate 202.

The thickness 209 of aluminum layer 206 can vary depending on particular applications. In some embodiments, aluminum layer 206 is deposited to a thickness 209 that ranges from about 5 micrometers to about 10 micrometers. In some applications, after aluminum layer 206 is deposited, surface 208 undergoes one or more finishing processes, such as one or more polishing, buffing, abrasive blasting, and chemical etching operations, in order to form a textured, matte, smoothed or polished surface 208. However, in some cases such a surface finishing process is not necessary.

It should be noted that surfaces titanium and titanium substrates do not traditionally need a metal coating since titanium and its alloys are generally known for their hardness and corrosion resistance. Thus, adding aluminum layer 206 on a titanium substrate 202 is not considered in conventional applications. This is understandable, given that aluminum can be softer and less corrosion-resistant than titanium and its alloys, and therefore offers no obvious benefits in conventional applications.

At 2D, at least a portion of aluminum layer 206 is converted to aluminum oxide layer 210. In some embodiments, an anodizing process is used, whereby at least a portion of aluminum layer 206 is electrolytically oxidized to a corresponding aluminum oxide. Since anodizing is a conversion process, surface 212 of aluminum oxide layer 210 replaces surface 208 of aluminum layer 206. Surface 212 of aluminum oxide layer 210 generally has a geometry corresponding to that of surface 208 of aluminum layer 206. That is, a smooth and polished surface 208 of aluminum layer 206 can result in a smooth and polished surface 212 of aluminum oxide layer 210. Likewise, a textured and matte surface 208 of aluminum layer 206 can result in a textured and matte surface 212 of aluminum oxide layer 210.

Thickness 214 of aluminum oxide layer 210 will depend on particular applications. In some applications, aluminum oxide layer 210 is grown to a thickness 214 that ranges from about 8 micrometers and 30 micrometers. In some cases, thickness 214 ranging from about 5 micrometers to about 20 micrometers is found to be thin enough to provide an aluminum oxide layer 210 with good visible clarity (i.e., little cloudiness), yet thick enough to provide good abrasion resistance. The hardness of aluminum oxide layer 210 can vary depending, in part, on the anodizing conditions. In some embodiments, a “Type II anodizing” (as defined by Mil-A-8625) process is used to form aluminum oxide layer 210 having a hardness of about 300 HV or greater.

In some embodiments, only a portion of aluminum layer 206 is converted to aluminum oxide layer 210. In other cases, substantially all of aluminum layer 206 is converted to aluminum oxide layer 210. In some cases, all of aluminum layer 206 is converted to aluminum oxide layer 210, and also a portion of substrate 202 is converted to a corresponding oxide layer. These embodiments are described below with reference to FIGS. 3A-3C, showing cross-section views of interface portion 216 of part 200.

FIG. 3A shows interface portion 216 of part 200 after an anodizing process has converted a portion of aluminum layer 206 to aluminum oxide layer 210. In some cases, the anodizing forms pores 302 within aluminum oxide layer 210. The size of pores 302 will depend, in part, on the conditions of the anodizing process, as will the hardness and cosmetics of aluminum oxide layer 210. If a Type II anodizing process is used, which involves anodizing in a sulfuric acid electrolyte, pores 302 can have diameters ranging from about 10 nanometers (nm) and about 50 nm. In one embodiment, the Type II anodizing is performed using a voltage of about 15 V, resulting in pores 302 having a diameter of about 30 nm. A Type II anodizing process can result in aluminum oxide layer 210 being substantially colorless and transparent, which allows the remainder of aluminum layer 206 to be visible through aluminum oxide layer 210. Thus, any surface texturing applied to aluminum layer 206 (e.g., at FIG. 2C described above) may be visible to a user of part 200. For example, a polished and shiny surface of aluminum layer 206 would be visible through aluminum oxide layer 210. Likewise, a textured or matte surface of aluminum layer 206 would be visible through aluminum oxide layer 210.

As described above, in some cases a very thin intermediate metal layer (not shown), such as a 2-6 nanometer thick layer of silver, is positioned between aluminum oxide layer 210 and titanium oxide layer 304. It is preferable for the intermediate layer to be composed of a metal that is inert to the anodizing process (i.e., does not chemically react with and contaminate the anodizing bath). The anodizing process can then terminate at the intermediate layer, making a top surface of the intermediate layer visible. In some cases, the intermediate layer is composed of a highly reflective metal, such as silver, thereby providing a shiny and cosmetically appealing underlying surface that is visible through aluminum oxide layer 210.

FIG. 3B shows part 200 after the anodizing process is allowed to proceed until substantially all of aluminum layer 206 is converted to aluminum oxide layer 210. When all of aluminum layer 206 is consumed by oxidation, substrate 202 starts to anodize at the terminuses of pores 302. If substrate 202 is composed of titanium or a titanium alloy, a corresponding titanium oxide layer 304 forms between aluminum oxide layer 210 and substrate 202. A voltage limit can control the extent of substrate 202 oxidation. In one example, when a Type II anodizing of aluminum layer 206 is conducted under galvanostatic control with an applied current density of about 1.5 Amps/dm², then the potential will start to rise (from an approximate value of 15 V) upon complete conversion of aluminum layer 206 to aluminum oxide layer 210. From this point onwards, the titanium substrate 202 can be oxidized to titanium oxide layer 304, which is a pore-free barrier layer type film, having a thickness proportional to the applied voltage.

At FIG. 3C, the anodizing process is allowed to proceed until titanium oxide layer 304 achieves a desired thickness 306. Unlike aluminum oxide layer 210, titanium oxide layer 304 is a non-porous, barrier layer type film and nominally reaches up to tens or hundreds of nanometers in thickness 306. Anodizing of titanium is a self-limiting process, based on the anodizing voltage used, with higher voltages generally resulting in titanium oxide layers having greater thicknesses. In some embodiments, thickness 306 of titanium oxide layer 304 is chosen to provide a predetermined color to part 200 by thin film interference effects. In general, thin film interference coloring occurs when light is reflected by boundaries of a thin film of a transparent or partially transparent material. To illustrate, optical pathways for incident light ray 308 are illustrated in FIG. 3C. Incident light ray 308 enters and passes through aluminum oxide layer 210. A first portion of light ray 308 reflects off interface surface 310 between aluminum oxide layer 210 and titanium oxide layer 304, and is reflected as light ray 314. A second portion of light ray 308 travels through aluminum oxide layer 210 and is reflects off interface 312 (which can also be referred to as the surface of underlying substrate 202) between titanium oxide layer 304 and substrate 202, and is reflected as light ray 316.

The different optical paths of reflected light rays 314 and 316 causes constructive and destructive interference between reflected light rays 314 and 316, with the degree of constructive and destructive interference depending upon differences in their phases. The difference in the phases, in turn, depends on the degree of thickness 306 of titanium oxide layer 304. In this way, the light interference can impart a perceived color to titanium oxide layer 304, and thereby provided a colored coating to part 200. Thus, the term “interference color” can refer to a color imparted to a thin film by thin film interference effects, including first order and second order interference. The resulting color can depend not only on the thickness of the thin film, but also the material of the thin film. Thus, for example, a titanium oxide thin film may be characterized as having a different range of colors compared to an aluminum oxide thin film. Other factors that may affect a perceived color can include the color of the underlying substrate 202, which will depend on the composition of metal(s) within the substrate 202.

Note that differences in the indices of refraction of aluminum oxide layer 210 and titanium oxide layer 304 can also cause some amount of diffraction of incident light 308. However, the thin film interference coloration provided by titanium oxide layer 304 is still apparent and observable through aluminum oxide layer 210 as long as aluminum oxide layer 210 and titanium oxide layer 304 are sufficiently transparent. In addition, variations of the thickness of aluminum oxide layer 210 may not be cosmetically apparent where aluminum oxide layer 210 is nominally transparent to visible light. This allows for greater tolerance with regard to thickness variations of aluminum oxide layer 210.

As described above, the interference coloration provided by titanium oxide layer 304 depends on thickness 306, which, in turn, can be tuned by controlling the applied potential used during the anodizing process. For example, in a particular embodiment, a 30-40 V potential results in a 50-60 nm thick titanium oxide layer 304, resulting in imparting a blue interference color to part 200. Examples of other anodizing voltage potentials used to form different interference colors are described below with reference to FIG. 4. In some applications, thickness 304 ranges between about 20 nm to about 150 nm.

It should be noted that the interference coloring provided by titanium oxide layer 304 is colorfast in that the color cannot be washed away or faded, for example, by exposure to ultraviolet (UV) light, as long as the interfaces 310 and 312 of titanium oxide layer 304 remain substantially intact. This is in contrast to oxide films colorized by dyes or pigments, which can in some cases be susceptible to fading by UV light or by exposure to certain chemicals.

As noted above, titanium oxide layer 304 is nominally very thin and, by itself, can be prone to abrasion and scratching. Aluminum oxide layer 210 addresses this issue by covering and protecting titanium oxide layer 304 from exposure to forces that could otherwise abrade or scratch titanium oxide layer 304. The hardness of aluminum oxide layer 304 will depend on a number of factors, such as the anodizing process parameters and the type of aluminum alloy anodized. As described above, in some embodiments, aluminum oxide layer 210 can have a hardness of about 300 HV or higher, which can provide substantial protection for may consumer product applications. Furthermore, if aluminum oxide layer 210 does become partially eroded, the integrity of titanium oxide layer 304 can be preserved, and therefore may not significantly change the visual appearance of part 200. That is, interface surfaces 310 and 312, which dominates the optical reflections for thin film interference coloring, and hence the visual appearance of part 200, can remain undamaged since they can be micrometers below the surface of aluminum oxide layer 210. In these ways, aluminum oxide layer 210 preserves the aesthetics of titanium surface finishes, such as intense and colorfast interference coloring of titanium oxide layer 304.

In addition, because the overlying aluminum oxide layer 210 is porous, it has the capacity to absorb colorants, which may be sealed in as per any suitable coloring techniques, such as infusing one or more of dye, pigment and metal within pores 302. This colorizing of aluminum oxide layer 210 can be used to adjust the color of the inference colorized titanium oxide layer 304. That is, the color imparted by a colorant within pores 302 can combine with the color imparted by interference coloring of titanium oxide layer 304 to accomplish a predetermined color to part 200, which may not be achievable through interference coloring or coloring of aluminum oxide layer 210 alone. For example, a UV-stable red organic dye may be infused within pores 302 of aluminum oxide layer 210 to achieve red colors, which may not be achievable by interference coloring alone. As another example, a UV-stable gray or black dye may be used to change the lightness of an interference color of titanium oxide layer 304. It should be noted, however, that colorizing of aluminum oxide layer 210 is optional and may not be used in some applications.

FIG. 4 shows a chart indicating a relationship between anodizing voltage potential and interference color of a titanium oxide layer, as measured by a* and b* color opponent dimension values in accordance with CIE 1976 color space model. In general, a* values represent amounts of green and red/magenta, and b* values represent amounts of blue and yellow of a sample. Negative a* values indicate a green color while positive a* values indicate a red or magenta color. Negative b* values indicate a blue color and positive b* values indicate a yellow color. In the chart of FIG. 4, b* values are represented on the horizontal axis and a* values are represented on the vertical axis.

As described above, the thickness of a titanium oxide layer is directly dependent upon the voltage used during the anodizing process. Thus, the interference color of the titanium oxide layer directly correlates with the applied anodizing voltage. The chart of FIG. 4 indicates that the interference color of a titanium oxide layer will cycle through hues between yellow, blue, green and red depending on the applied voltage used during the anodizing process. For example, a 10-volt potential (corresponding to a titanium oxide thickness of about 15-20 nm) used during anodizing of titanium results in a brown color, and a 40-volt potential (corresponding to a titanium oxide thickness of about 50 nm) results in a light blue color. In some embodiments where a neutral is preferred, a titanium oxide thickness of about 50-60 nm may be preferred since the resulting light blue color is very slight and the blue color can approximate a metallic appearance of the underlying substrate. However, the titanium oxide layer can have any suitable thickness and hue. According to some embodiments, the thickness of the titanium oxide layer is between about 20 nm and about 150 nm.

FIGS. 5A and 5B show a plan view of part 500, which includes a coating 502 having interference coloring, in accordance with some embodiments. FIG. 5B shows a partial cross-section A-A of part 500. Part 500 can correspond, for example, to an enclosure, or part of an enclosure, for an electronic device. Coating 502 corresponds to an oxide material formed over substrate 512, which can be composed of a titanium or titanium alloy.

Coating 502 includes first portion 504, which is characterized as having a color by interference coloring, and second portion 506, which does not have interference coloring. In particular, second portion 506 includes aluminum oxide layer 510, which can be a porous anodic film, such as aluminum oxide layer 210 described above with reference to FIGS. 2A-2D and 3A-3C. Aluminum oxide layer 210 can be transparent, or partially transparent, such that substrate 512 is viewable through aluminum oxide layer 210. First portion 504 includes aluminum oxide layer 210 as well as titanium oxide layer 514. Titanium oxide layer 514 is sufficiently transparent and has a thickness suitable for providing a color to first portion 504 via interference coloring. Thus, the length and shape of titanium oxide layer 514 defines boundary 508 of first portion 504. In some embodiments, boundary 508 has a shape in accordance with a logo, writing or other cosmetic or identifying feature.

Coating 502 can be formed using the aluminum layer deposition and anodizing processes described above with reference to FIGS. 2A-2D, with the addition of a masking process. For example, after an aluminum layer is deposited, a first anodizing process can be used to form aluminum oxide layer 510 of both first portion 504 and second portion 506. After substantially the entire aluminum layer is converted to aluminum oxide layer 510, second portion 506 can be masked. Then, first portion 504 is exposed to a second anodizing process, whereby a portion of substrate 512 is converted to titanium oxide layer 514. Note that in some embodiments, it may be desirable to colorize aluminum oxide layer 510 (e.g., using dye, pigment or metal) in one or both of first portion 504 and second portion 506.

FIG. 6 shows flowchart 600 indicating a process for forming a coating on a titanium substrate, in accordance with some embodiments. At 602, a surface of the titanium substrate is optionally finished using, for example, one or more of a buffing, polishing, abrasive blasting and chemical etching processes. The titanium substrate can be composed of a titanium alloy, such as a Ti-6Al-4V titanium alloy, or may be composed of substantially pure titanium.

At 604, an aluminum layer is deposited onto the surface of the titanium substrate. The aluminum layer can be composed of an aluminum alloy or substantially pure aluminum. The aluminum layer can be deposited using any suitable technique. In some cases, the aluminum layer is deposited using one or more physical vapor deposition (PVD), hot dip aluminizing, cladding and electroplating processes. At 606, a surface of the aluminum layer is optionally finished using, for example, one or more of a buffing, polishing, abrasive blasting and chemical etching processes.

At 608, at least a portion of the aluminum layer is converted to an aluminum oxide layer. In some embodiments, an anodizing process is used, whereby the resulting aluminum oxide layer will have pores. In some applications, it is preferable that the aluminum oxide layer be mostly transparent to visible light and also has a hardness of at least about 300 HV. In a particular embodiment, a Type II anodizing process is used, whereby the anodizing process is performed in an electrolytic bath having sulfuric acid, resulting in a substantially transparent, cosmetically appealing aluminum oxide layer. In some embodiments, substantially the entire aluminum layer is converted to a corresponding aluminum oxide layer. Complete conversion of the aluminum layer will be apparent by a rise in potential during the anodizing process if conducted under current control, or by a fall in current if the process is conducted under voltage control.

At 610, a portion of the titanium substrate is optionally converted to a titanium oxide layer. These embodiments involve anodizing through the entire aluminum layer, and continuing the anodizing process until a portion of the titanium substrate is converted to a corresponding titanium oxide layer. In some embodiments, the part remains within the same electrolyte as used to anodize the aluminum layer. In other embodiments, a different electrolyte is used to anodize the titanium layer versus the aluminum layer. The titanium oxide layer is a barrier type film that is non-porous. That is, the aluminum oxide layer formed by anodizing will have more pores than the titanium oxide layer since the titanium oxide layer has substantially no pores formed by the anodizing process. In some cases, the anodizing voltage is chosen based on a desired interference coloring of the titanium oxide layer. In some embodiments, the titanium oxide layer has a thickness ranging from about 20 nm and about 150 nm.

At 612, a colorant is optionally deposited within the aluminum oxide layer. The colorant can include, for example, a dye, pigment or metal material that is infused within pores of the aluminum oxide layer. A color of the colorant can combine with the interference coloring provided by the titanium oxide layer, resulting in a final color that may not be achievable using standard coloring techniques. At 614, pores of the aluminum oxide are optionally sealed in order to improve the corrosion resistance of the coating and/or to seal in any colorant within the pores. Any suitable sealing process can be used, including suitable hydrothermal aqueous-based anodic film sealing techniques.

It should be noted that whilst the preceding description focuses on titanium and its alloys as a substrate metal, the processes described herein may also be applied to any of a number of other suitable metals and their alloys that yield similar thin nonporous oxides under certain anodizing conditions. Such metals can include one or more of zirconium, tantalum, hafnium, niobium and alloys thereof. In some cases, the substrate includes titanium and one or more of zirconium, tantalum, hafnium and niobium. In addition, chromium oxide or other metal oxides based on at least one of rhenium, molybdenum, cobalt, or tantalum may provide a wide range of interference colors. Thus, the term “interference colors” can refer to a range of colors, which are formed because of the thin film interference effect. The interference colors can include first order, or second order interference colors such as yellow, orange, pink, purple, blue, or green hues.

The foregoing description, for purposes of explanation, uses specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. 

What is claimed is:
 1. A part, comprising: a substrate composed of titanium or a titanium alloy; and a coating disposed on the substrate, the coating including an aluminum oxide layer.
 2. The part of claim 1, wherein the coating includes a titanium oxide layer.
 3. The part of claim 2, wherein the aluminum oxide layer is more porous than the titanium oxide layer.
 4. The part of claim 2, wherein the titanium oxide layer has a thickness ranging from about 20 nm to about 150 nm.
 5. The part of claim 2, wherein the titanium oxide layer imparts a color to the coating via thin film interference coloring.
 6. The part of claim 2, wherein the aluminum oxide layer has a colorant infused within pores of the aluminum oxide layer.
 7. The part of claim 1, wherein the aluminum oxide layer has a colorant infused within pores of the aluminum oxide layer.
 8. The part of claim 1, wherein the aluminum oxide layer has a hardness of about 300 HV or greater.
 9. The part of claim 1, wherein the coating includes a layer of aluminum or aluminum alloy between the aluminum oxide layer and the substrate.
 10. The part of claim 1, wherein the aluminum oxide layer has a thickness ranging from about 5 micrometers to about 20 micrometers.
 11. A method of forming a coating on a substrate, the substrate composed of titanium or a titanium alloy, the method comprising: depositing a layer of aluminum or an aluminum alloy on the substrate; and converting at least a portion of the layer of aluminum or the aluminum alloy to an aluminum oxide layer.
 12. The method of claim 11, wherein substantially all of the layer of aluminum or aluminum alloy is converted to the aluminum oxide layer.
 13. The method of claim 12, wherein at least a portion of the substrate is converted to a titanium oxide layer.
 14. The method of claim 13, wherein the aluminum oxide layer is more porous than the titanium oxide layer.
 15. The method of claim 13, wherein the titanium oxide layer has a thickness ranging from about 20 nm to about 150 nm.
 16. The method of claim 11, further comprising infusing a colorant within pores of the aluminum oxide layer.
 17. An enclosure for an electronic device, the enclosure comprising: a substrate composed of titanium or a titanium alloy, the substrate having a coating corresponding to an outer surface of the enclosure, the coating including an aluminum oxide layer.
 18. The enclosure of claim 17, wherein the coating includes a titanium oxide layer.
 19. The enclosure of claim 18, wherein the titanium oxide layer has a thickness ranging from about 20 nm to about 150 nm.
 20. The enclosure of claim 19, wherein the enclosure includes a first portion that is interference colored and a second portion that is not interference colored, wherein the titanium oxide layer is part of the first portion. 